Peter Kenyon, TunnelTalk

As the first water finally flows through Seattle's problem-hit 21km (13-mile) Brightwater Conveyance Tunnel, project owner King County moves to advanced design of the Fremont Siphon Replacement Program.

Project scope calls for twin 60in microtunnels to be driven under the Lake Washington Ship Canal adjacent to the existing Fremont Siphon pipes that are housed in a century-old concrete tunnel (Fig 1). Also included is connection to sewer lines on either side of the ship canal and sinking retrieval and launch shafts for the microtunnel machines.

Fig 1. Alignment of Fremont Siphon Replacement microtunnels

Four options were considered at the initial design stage, including rehabilitation of the existing tunnel and a TBM-driven 15ft (4.5m) diameter replacement tunnel, but advanced design will focus on the microtunnel solution.

The Fremont Siphon is a critical conveyance line that carries wastewater from the county's northeastern service area to the West Point Treatment Plant. The siphon pipelines and concrete tunnel, constructed in the early 1900s, carry up to 220 million gal/day during storms, but are now considered to be at the end of their design life.

"The replacement project is still in design," King County Wastewater Treatment Division (WTD) spokeswoman Annie Kolb-Nelson told TunnelTalk. "We are currently reviewing bid proposals for a construction manager to do a constructability review during this design phase and ultimately provide construction management services during construction. MWH Global is the prime design consultant."

She added: "For a variety of different reasons, not just cost, WTD is moving forward with construction of the new siphon via two microtunneled pipelines. These reasons include potential impacts to the community, potential damage to the existing siphon during rehabilitation work, potential code issues associated with the location of the existing siphon, ground conditions, and proximity to the bottom of the Lake Washington Ship Canal."

Meanwhile contractor J.W. Fowler of Oregon is excavating a 2,000ft long tunnel, using a small-bore 2.6m-diameter Herrenknecht EPBM, for another siphon replacement program for King County at Ballard, also under the Lake Washington Ship Canal. The TBM was launched earlier this month, and is expected to hole through into the reception shaft by February 2013.

Brightwater Tunnel complete
News of the latest sewer upgrades in Seattle comes as the first water flows through King County's US$1.8 billion Brightwater Conveyance Tunnel mark the completion, after six long years, of the region's largest clean water project in half a century.

Contractor Jay Dee/ Collucio (JDC) had successfully completed excavation of the four-mile Brightwater West Tunnel in early 2010, when owner King County had to address the Central Tunnel contractor's inability to complete that tunnel on time after its slurry TBM suffered serious damage.

With the original contractor, the Vinci/Parsons/Frontier-Kemper JV (VPFK), already several months behind schedule owing to a very high level of hyperbaric interventions forced by extreme cutter wear in difficult ground conditions, King County Executive Dow Constantine took the tough decision to select JDC in April 2010 to complete the remaining 1.9 miles of the four-mile-long Central Tunnel.

Lovat TBM breaks through shell of stranded TBM

Final breakthrough was achieved in August 2011, 50 days ahead of a revised schedule, when JDC's Lovat EPBM, which had already been used to successfully excavate the West Tunnel, made its final push through a concrete plug and into the shell of VPFK's stranded TBM, thereby completing the problematic Central Tunnel.

"Meeting this critical milestone fulfils the promise we made to ratepayers a decade ago to integrate the need for more wastewater treatment capacity with the regional environmental goal of protecting Puget Sound," said Constantine as the first water flowed through the new tunnel earlier this month (November 2012).

The 600ft-deep outfall, which rests on the floor of Puget Sound a mile off Point Wells, serves as the primary discharge point for the Brightwater Treatment Plant serving both south Snohomish County and north King County. The 13 mile-long tunnel from the treatment plant north of Woodinville to Point Wells conveys highly treated wastewater to the outfall.

"We are proud of what this new treatment system will accomplish to protect the environment and the economy for our next generation of Puget Sound area residents," said Constantine.

The successful completion of the complex Brightwater conveyance tunnels last year earned prestigious international honours for JDC and King County's project management team.

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